Heat-developable light-sensitive materials are well known in the art. Examples of heat-developable light-sensitive materials and heat development processes are described, for example, in Shashinkoqaku no Kiso, "Edition of Higin-en Shashin", pages 242 to 255 (Corona Co., Ltd., 1982) and U.S. Pat. No. 4,500,626.
With respect to processes for obtaining positive color images many methods have been proposed. For instance, in U.S. Pat. No. 4,559,290, a method is described where an oxidized compound, which in its oxidized state does not have a dye releasing ability, obtained by converting a so-called DRR compound, is present with a reducing agent, the reducing agent is oxidized by exposed silver halide upon heat development, and the oxidized compound is reduced with the remaining reducing agent which is not oxidized, whereby a diffusible dye is released. Further, in European Patent Application (OPI) No. 220,746 and Kokai Giho 87-6199 (Kokai Giho, Vol. 12, No. 22), a method of forming a heat-developable positive image using a novel compound which can release a diffusible dye by a similar mechanism is described.
In the process of forming positive images using the reducible dye providing compound as described above, an electron donor and an electron transfer agent are ordinarily employed together as reducing agents.
It is known that 1-phenyl-3-pyrazolidinone derivatives which are known developing agents are effective as electron transfer agents (ETA) used in such a system. However, when a 1-phenyl-3-pyrazolidinone derivative, for example, 1-phenyl-3-pyrazolidinone or 4-methyl-4'-hydroxymethyl-1-phenyl-3-pyrazolidinone is employed as an electron transfer agent, it has been found problems occur such as increase in stain and degradation of an S/N ratio of image after storage, since the electron transfer agent is unstable and gradually decomposes and disappears during storage.